This invention relates to a suspension system for a superconducting magnet assembly for a magnetic resonance imager (hereinafter called "MRI") and in particular to the suspension for an open architecture magnet assembly.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen cryocoolant means. The extreme cold ensures that the magnet coils are superconducting, such that the coils can be operated in the persistent mode, that is when a power source is initially connected to the coil for a relatively short period of time to introduce a current flow through the coils and a superconducting switch is then closed, the current will continue to flow, thereby maintaining the coil current and a magnetic field. Superconducting magnets find wide application in the field of MRI.
Considerable research and development efforts have been directed at reducing magnet heat load and eliminating the need to continuously replenish the boiling liquid helium. While the use of liquid helium to provide cryogenic temperatures has been widely practiced and is satisfactory for MRI operation, the provision of a steady supply of liquid helium to MRI installations all over the world has proved to be difficult and costly. This has led to considerable effort being directed at superconducting materials and magnet structures in which the helium is recondensed and reused. This leads to the need for good thermal insulation between the cryostat and the ambient temperature.
Another problem encountered by most MRI equipments is that they utilize solenoidal magnets enclosed in cylindrical structures with a central bore opening for patient access. However, in such an arrangement, the patient is practically enclosed in the warm bore, which can induce claustrophobia in some patients. The desirability of an open architecture structure in which the patient is not essentially totally enclosed has long been recognized. Unfortunately, an open architecture structure poses a number of technical problems and challenges. One problem is to provide a suitable support structure which occupies much less space than conventional support structures, and yet which nevertheless can support the magnet assembly under the considerable electromagnetic forces and thermal forces encountered during operation.
The suspension system of an MRI magnet has to support the magnet mass, while providing adequate stiffness with minimal conduction heat leak. In addition to the mass and its dynamic load, an open architecture MRI suspension must support a large electromagnetic (EM) net force in each half of the magnet in the axial direction as well as a possible transverse EM force due to misalignment. The stiffness requirement in all directions is also more demanding in such an assembly in order to provide field stability under vibration. Moreover, all of the structural requirements should be met without significant increase of conduction heat leak through the suspension system. Good heat interception in the suspension is essential to bring the 4 K heat load within the cooling capacity of the magnet, particularly if helium recondensing is provided since the mechanical cryocooler in a recondensing system is frequently at, or near, its cooling capacity. Still further, the system has to be designed to accommodate the difference of thermal differential expansion and contraction between the aluminum helium vessel and the suspension system without complex and expansive machining.
All of the overlapping and conflicting requirements must be satisfied for a practical and satisfactory MRI superconducting magnet structure.
Thus, there is a particular need for a superconducting magnet structure and support assembly which overcomes the aforementioned problems while providing good mechanical support to resist the strong magnetic forces, along with good thermal insulation and isolation.